1. Field of the Invention
The present invention relates to methods and apparatus for frequency stabilizing lasers. In particular, the present invention relates to lasers stabilized by dual etalons.
2. Discussion of Background Art
Lasers are used in many applications where the oscillation frequency of the device is utilized as a clock. At the extreme end of applications the demonstration of stabilities better than 1 part in 1014 enables lasers to be used to synchronize clocks worldwide for highly precise time measurements. These systems require an extremely high degree of isolation against environmental disturbances (such as temperature variations and vibrations) since even minute changes in the length of laser cavities causes variations in the laser frequency. Stable lasers are also required in applications such as coherent laser radar systems. In these systems laser pulses are sent from a sensor location to a target that may be many km distant and the change in phase of the signal upon return to the sensor is used to measure properties of the target. Such measurements rely on measuring the phase very accurately by heterodyning the return signal with a local oscillator beam and comparing that phase with a similar measurement carried out on a sample of the transmitted laser pulse. If there is a change in the local oscillator frequency while the pulse is in transit to the target and back, these phase measurements can become invalid. For an order of magnitude estimate of stability requirements in these circumstances it is noted that phase errors must be <<π radians over the round-trip time t=2R/c, where R is the target range and c is the speed of light=3·108 m/s. For a target range of 50 km the round-trip time is 0.33 μsec giving an angular frequency stability requirement of <<π/0.33 ms, or a frequency stability of Δf=1.5 kHz. For a laser with an emission wavelength λ=1.5 μm the frequency is given by f=c/λ=2·1014 Hz, thus leading to a fractional frequency stability requirement of Δf/f=7.5·10−12. To further put this into context the frequency of a laser determined by a standing wave formed in an optical cavity of length L whose resonant frequency is a multiple m of the quantity c/2 L (assuming the cavity is a vacuum). Changing the cavity length by a small amount ΔL causes a frequency deviation magnitude given by |Δf|/f=ΔL/L. For a cavity length of 1 cm, a frequency of 2·1014 Hz, and a frequency stability requirement of 1.5 kHz, the tolerance on the length ΔL is then 7.5·10−14 m, or 0.000075 nm, an extraordinarily small number given that, for example, the diameter of a hydrogen atom is approximately 0.1 nm.
Over the years techniques have successfully been developed to build lasers with frequency stabilities to meet these stringent demands. This is generally not done by directly stabilizing the laser and its environment, but rather by active means whereby a highly stable and environmentally isolated “frequency reference” is created such that the laser emission frequency can be stabilized to the frequency reference. Although a number of techniques currently exist the perhaps best known is the so-called PDH technique named after Pound, Drever, and Hall who pioneered the technique for microwave signals and transferred these developments to laser cavities. The invention disclosed herein applies equally well to all techniques that rely on locking a laser to an etalon. Alternatives to the PDH technique include, but are not limited to, polarization locking (see for example T. W. Hansch, B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Comm., Vol 35, 3, 441-444, 1980) and tilt locking (see for example B. J. J. Slaggmolen et al. “Frequency Stability of Spatial Mode Interference (Tilt) Locking”, IEEE Journal of Quantum Electronics, vol. 38, no. 11, November 2002).
The PDH technique is very robust in many ways, such as stability against intensity fluctuations of the laser. However, it and other techniques like it, suffer from one significant limitation. While the technique works quite well under the relatively stable conditions that can be created in laboratory environments it is not well suited to harsh environments. Essentially the PDH technique involves locking the laser to a very narrow “fringe” in an etalon. Small excursions from the center of the fringe result in error signals that are used to change the laser frequency in such a manner that it brings the frequency back to the center of the fringe. The problem is that the higher the frequency stability requirement, the narrower the fringe has to be. This has the inadvertent effect of reducing the range over which the error signal is valid. As a result, the higher the frequency stability requirement, the easier it is to “kick” the error correcting servo control system out of lock. When that happens a search has to be carried out to find the correct fringe again. During this search time the laser frequency is not stabilized and may not be valid as a clock for the measurement system. To further compound difficulties, it is possible for the servo electronics to lock onto erroneous fringes during the search so that the frequency sought is not the correct frequency.
Under laboratory conditions these problems can typically be dealt with using complex equipment and techniques. However, coherent laser radar and other systems often operate in very difficult conditions where complexity must be avoided and where autonomous operation with high reliability is essential. An example would be a system mounted to a fighter aircraft that subjects the laser to high vibrations, shock, and wide temperature swings, all the while operating in a highly confined space with no possibility of user intervention. Under these conditions it is essential to have a laser that locks itself very reliably and has sufficient smart controls to re-lock itself if the initial lock condition is disturbed.
Multiple etalons have been used with lasers prior to this invention but not for the purpose intended here. For example U.S. Pat. No. 4,947,398 to Yasuda describes a laser that utilizes two etalons inserted into the laser cavity to narrow the width of the laser line. That arrangement of using intra-cavity etalons would not be appropriate for achieving a high degree of frequency control. The Yasuda patent is concerned with lasers that have linewidths on the order of 1 nm. This corresponds to a frequency error on the order of 1,000 GHz, which is typically 8-10 orders of magnitude greater than the frequency stability we are concerned with. Another use of multiple etalons is as “clean-up cavities” prior to locking using e.g. the PDH technique. Clean-up cavities are used to strip small amounts of transverse modes from a main laser mode (normally TEM00). Higher order transverse modes represent noise at discrete frequencies and can be stripped off from the laser beam using a high-finesse etalon. As an example, the document “Simulation of Input Optics with LIGO End-to-End Model” by S. Klimenko et al. available at www.phys.ufl.edu/˜klimenko/ionote.pdf notes that the pre-cleaning etalon used in the LIGO (Laser Interferometer Gravitational Wave) system acts as a low pass frequency filter with a bandwidth of about 3 kHz.
A need remains in the art for improved apparatus and methods for stabilizing lasers under non-laboratory conditions.